The present invention generally relates to signal detection in wireless communications networks, and in particular to wireless network architectures that utilize signal measurements from multiple cells for positioning, locating, and location-based services.
In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Some radio access technologies have the capability of identifying user geographical location in the network, e.g., discerning or determining the geographical location of a wireless terminal or user equipment unit (UE). The ability to determine geographical location has facilitated or enhanced a large variety of commercial and non-commercial services. Such services include, by way of example, navigation assistance, social networking, location-aware advertising, emergency calls, etc.
Of the services that utilize or capitalize upon geographical location, different services may have different positioning accuracy requirements. These differing positioning accuracy requirements may be imposed by the particular application that provides the respective service. In addition, some countries have specific regulatory requirements relating to positioning accuracy for basic emergency services, such as (for example) FCC E911 in the United States of America. Such governmental or other regulatory requirement(s) may impose additional constraints on the desired quality of the positioning service.
Currently there exists a wide range of positioning methods. Many of the current positioning methods in one or another way involve timing measurements. Furthermore, some of the current positioning methods are based on a multilateration technique. The multilateration technique is a way to determine a geometrical position from intersection of multiple surfaces, e.g., spheres or hyperboloids. Such an intersectional approach requires measurements from multiple sites with good geometry. In fact, for an intersectional approach ideally at least three such sites are necessary to determine a two-dimensional position and four sites to determine a three dimensional position. In practice these requirements mean that a user equipment unit (UE) needs to measure significantly more cells because some of them are co-located or have bad geometry.
FIG. 1A and FIG. 1B illustrate, at least in part, a downlink Observed Time Difference Of Arrival method (OTDOA) method which has been standardized by 3GPP for LTE. In FIG. 1A each hyperbola illustrates an area with a same level of the reference signal time difference (RSTD) for two base stations. The terminal (e.g., wireless terminal) measures the timing differences of multiple base stations. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias. In FIG. 1B intersection of three hyperbolic stripes provides an estimation of a wireless terminal location. In order to solve for position, precise knowledge of the transmitter locations and timing is needed. With OTDOA, unlike with measuring time of arrival (TOA), synchronization between base stations and terminals is not needed.
To enable positioning in LTE and to facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning (positioning reference signals, or PRS) have been introduced and specific positioning subframes have been agreed in 3GPP. See, e.g., 3GPP TS 36. 211 (Rel-9, B), which is incorporated herein by reference. It is, however, left up to user equipment unit (UE) to decide whether to use or not PRS for positioning measurements.
At least on the downlink LTE uses orthogonal frequency division multiplexing (OFDM), wherein data is simultaneously encoded over various sub-carriers. A data stream is split into N parallel streams of reduced data rate and each parallel stream is transmitted on a separate sub-carrier. When the subcarriers have appropriate spacing to satisfy orthogonality (e.g., the sub-carriers' frequencies differ from each other by integer multiples of the base (lowest) sub-carrier frequency), the carriers are mutually orthogonal to each other and their spectra overlap. FIG. 2 illustrates a time-frequency plane of an Orthogonal Frequency Division Multiplexing (OFDM) system wherein symbols are modulated onto orthogonal time-frequency units (illustrated by way of example as the squares of FIG. 2) defined by the sub-carriers of an OFDM symbol.
In accordance with the 3GPP agreements, Positioning Reference Signals (PRS) are transmitted from one antenna port (R6) according to a pre-defined pattern. The specified PRS pattern for the case when one or two Physical Broadcast Channel (PBCH) antennas are in use is shown in FIG. 2. In FIG. 2 squares labeled “R6” indicate PRS resource elements within a block of 12 subcarriers over 14 OFDM symbols (e.g., 1 ms subframe with normal cyclic prefix). A set of frequency shifts can be applied to the pre-defined PRS patterns to obtain a set of orthogonal patterns which can be used in neighbor cells to reduce interference on PRS and thus improve positioning measurements. The effective frequency reuse of six can be modeled in this way. The frequency shift is defined as a function of Physical Cell ID (PCI) as νshift=mod(PCI,6) PRS can also be transmitted with zero power, or muted.
To improve hearability of the physical reference signal (PRS), e.g., to allow for detecting the PRS from multiple sites and at a reasonable quality, positioning subframes have been designed as low-interference subframes. For example, it has also been agreed that (in general) no data transmissions are allowed in positioning subframes. As a result, in synchronous networks, PRS are ideally interfered only by PRS from other cells having the same PRS pattern index (i.e. same vertical shift νshift) and not by data transmissions.
In contrast to synchronous networks, in partially aligned asynchronous networks PRS can still be interfered by transmissions over data channel(s), control channel(s), or physical signals when positioning subframes collide with normal subframes. This interference effect can be minimized by partial alignment (e.g., by aligning the beginning of positioning subframes in multiple cells within ½ of a subframe with respect to some time base).
If the user equipment unit (UE) uses PRS for positioning in general but is not able to detect PRS for a cell, it will try to detect Common Reference Signals (CRS) and to perform Reference Signal Time Difference (RSTD) measurements based on the Common Reference Signals (CRS). However, a failure to detect PRS and then searching for the other signals of the same cell increases the cell detection time and may also degrade positioning measurements. This is because Common Reference Signals (CRS) in a typical case have worse hearability than PRS due to a lower effective frequency reuse (namely, 3-reuse when two transmit antennas are used for CRS).
FIG. 3 illustrates that Positioning Reference Signals (PRS) can be transmitted in pre-defined positioning subframes grouped by several consecutive subframes (NPRS), i.e. one positioning occasion, which occur periodically with a certain periodicity of N subframes. The periodicity N is the time interval between two positioning occasions. For example, FIG. 3 shows three different groups of positioning subframes, each group of positioning subframes comprising six subframes (NPRS=6), and a first subframe of each group of positioning subframes being separated from a first subframe of the next in time group of positioning subframes by N frames. The periods N specified in the 3GPP standard are 160, 320, 640, and 1280 ms, and the number of consecutive subframes NPRS can be 1, 2, 4, or 6 [see, e.g., 3GPP TS 36.211 v9.1.0, Mar. 30, 2010, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation, which is incorporated herein by reference].
Since (for OTDOA positioning) Positioning Reference Signals (PRS) signals from multiple distinct locations need to be measured, the user equipment unit (UE) receiver has to deal with some Positioning Reference Signals that may be much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the user equipment unit (UE) would need to search blindly for signals. Blind search would negatively impact the time and accuracy of the measurements.
To reduce blind searching and to facilitate measurements made by the user equipment unit (UE), the network transmits assistance data to the user equipment units. The assistance data which includes, among other information, a neighbor cell list with Physical Cell Identities (PCIs), the number of consecutive downlink subframes NPRS, the PRS transmission bandwidth, the expected time of signal arrival, etc. The standardized OTDOA assistance information is specified in 3GPP TS 36.355 v9.2.1 Jun. 6, 2010, Evolved Universal Terrestrial Radio Access (E-UTRA); LTE Positioning Protocol (LPP), which is incorporated herein by reference.
In conventional practice, a wireless terminal comprises a correlation unit which operates in the time domain to correlate a signal propagated through a radio channel with replicas of the positioning reference signal to obtain a correlation sum. A detector compares normalized output of the correlation unit with a threshold value to determine times at which the positioning reference signal is present. Then, assuming a high post-correlation signal to noise ratio (SNR), an estimated arrival time of the positioning reference signal over a particular path of the radio channel is determined from a minimum of the times for which the positioning reference signal is present, subject to constraints which pertain to received power.
As mentioned above, the detector of the wireless terminal compares normalized output of the correlation unit with a threshold value to determine times at which the positioning reference signal is present. Hopefully selection of the threshold value achieves an appropriate compromise between detection probability and probability of false alarms (e.g., false alarms in locating the arrival time of the positioning reference signal). False alarms are detrimental to positioning performance and are in most cases difficult to correct. In some cases it has been known to make assumptions regarding statistics of a noise term used by the correlation unit in obtaining its correlation sum. For example, it has been assumed in some prior art practice that the received signal consists of the desired signal plus additive Gaussian noise.